1. Field of the Invention
The present invention relates to an image processor and image processing program for binary processing (half tone processing) for converting multi-grayscale image data into binary data that allows half tone display, and more particularly to an image processor and image processing program that can convert low resolution image data into high image quality binary data.
2. Description of the Related Art
Facsimiles and copiers acquire multi-grayscale image data by reading images formed on such print medium as paper by a scanner unit, and converting this into binary data, which allows half tone display. A facsimile sends binary data via a communication line. A copier regenerates the image from the binary data by a print unit. Other than facsimile and copiers, a personal computer system also reads images of a print medium using a scanner function, converts the read multi-grayscale image data into binary data, and regenerates and displays the image on a display device, for example.
Particularly in the case of a facsimile, the generation of binary data with about a 200 dpi low resolution is demanded by standards, and a scanner unit hereof outputs multi-grayscale image data with about a 200 dpi low resolution accordingly. Therefore when the low resolution multi-grayscale image data acquired by a scanner unit is converted into binary data, it is necessary to maintain the gradation of a photo while improving the visibility of fine lines and characters.
Generally a fixed dither method, using a fixed threshold matrix, and an error diffusion method, are used for the binary processing of multi-grayscale image data. There are two types of fixed dither method: a dot concentrated type fixed dither method, where the low threshold values of the threshold matrix are arranged at the center part so that half tone dots or lines grow as the input grayscale increases, and a dot dispersed type fixed dither method, where the threshold values of the threshold matrix are dispersed. The dot concentrated type is called an AM (Amplitude Modulation) screen, since a half tone is expressed by the sizes of the half tone dots where dots are concentrated, and a dot dispersed type is called an FM (Frequency Modulation) screen, since an image is reproduced by the closeness in the spaces of isolated dots.
In the case of the dot concentrated fixed dither method, resolution decreases if the number of screen lines is decreased to improve the graduation, and gradation drops if the number of screen lines is increased to improve resolution, so when the low resolution image data is binarized, it is difficult to maintain the gradation of a photo while improving the visibility of fine lines and characters.
In the case of the dot dispersed type fixed dither method used for ink jet printers, for example, a half tone is expressed by the density of dispersed dots, so gradation improves, but in the case of low resolution image data, the missing of dots stands out, and the visibility of characters and fine lines cannot be assured.
The error diffusion method, on the other hand, is a processing of repeating binarization while propagating or diffusing errors between input image data generated by binarization and original data into peripheral pixels, and is known as a binary processing that can implement both gradation and resolution. Such an error diffusion method is disclosed, for example, in Japanese Patent Application Laid-Open No. H8-307680.